Hitherto, surface illuminating apparatuses have generally widely been used for the indirect illumination in a room, signboards, display devices of various kinds of apparatuses, and the like. Among the above surface illuminating apparatuses, there is well known an apparatus such that a fluorescent lamp as a line light source or an incandescent lamp as a point light source is arranged on the back surface of a light diffusing plate and the light illumination is executed.
However, in such a surface illuminating apparatus, since a light amount differs depending on a distance from a light source arranged on the back surface of the light diffusing plate, it is difficult to obtain a surface illumination of a uniform brightness. To solve the above drawback, there has been proposed a surface illuminating apparatus comprising a flat plate-like transparent plate and the light sources arranged on both sides of the transparent plate. According to such an illuminating apparatus, by brushing one surface of the transparent plate, a number of concave and convex portions are formed on one surface of the transparent plate, the light from a light source is diffused and reflected by the concave and convex portions to thereby obtain an almost planar light source. According to the above surface illuminating apparatus, although the uniformity is improved, a large amount of reflected lights by the concave and convex portions formed on the surface of the transparent plate are not used for illumination and the illuminance is small, and a variation in illuminance easily occurs. Therefore, there is a drawback such that a thickness of transparent plate cannot be made thin.